I. Overview of Polyurethane Type Polymers
Polyurethanes and polyureas are polymers formed by the chemical combination of a resin blend and isocyanates. Polyisocyanurates are similar polymers but are formed with a large excess of isocyanates. The resin blend may contain one or several functional organic species, which contain a minimum of two hydroxyl or amine groups per molecule. The size of these functional species may vary from monomeric, e.g., ethylene glycol or carbonyl diamine, to very large species with molecular weights of about 6000 grams per mole. The larger species may be started from polyalcohols or carboxylic diacids and grown, respectively, via polyether or polyester chain extension. Alternatively, a larger molecule may also be started from a carboxylic diacid and grown via polyether chain extensions. The final functional groups may be hydroxyl or amine and may be positioned on the terminal or secondary carbons. In addition to these functional molecules, the resin blend may also contain colorants, organic dyes or mineral pigments, and catalysts, Lewis acids including amines, metal salts and organo metallic compounds, and additives, to control leveling, sheen, flow, wet out, adhesion and moisture level.
The hardener side may be equally complex. It consists of organic species containing two or more reactive isocyanate groups per molecule. These may vary in size from simple monomers to higher homologues and may be generically of the classes called aromatic, aliphatic or pseudo-aliphatic compounds. Aromatic isocyanates are of a type, which, when reacted to make a polyurethane, will discolor under the effect of ultraviolet radiation while the aliphatic isocyanates are inherently more light stable. Pseudo-aliphatic isocyanates are aromatic species which incorporate within the molecular structure electron delocalizing subgroups which reduce the sensitivity of the molecule to photo-induced color changes.
There is an overriding characteristic of polyurethane type chemistry, to wit: If there is any moisture, water, present, the isocyanate will preferentially react with this moisture to generate carbon dioxide gas. This gas becomes entrapped within the polymerizing mixture, forming a foam. Due to the historic difficulty of removing moisture, this natural tendency for foam development was exploited and augmented with additional water and or other volatilizing agents. Hence, urethanes became the polymer of choice for manufacturing cellular plastics.
The historic method for making a non-cellular urethane type polymer involves a "two shot" approach where the polymer is premade to a 50 to 90% completion under very controlled conditions at a chemical plant. The value of this approach is that the majority of the isocyanate groups have already been reacted with a resin and are simply no longer available for subsequent water reaction. The disadvantage of this method is that the prepolymer formed is extremely viscous and can only be used when highly diluted with solvents. This approach is useful only for thin films and coatings; otherwise, the solvent becomes entrapped and generates bubbles.
Thus, historically, urethane technology has only been developed for foams or thin coatings.